1. Field of the Invention
This invention relates to a radiation image recording system in which a stimulable phosphor sheet is exposed to a radiation transmitted through an object to store the energy of the radiation and record a latent image of a radiation image in the form of stored energy of radiation, the stimulable phosphor sheet is then exposed to stimulating rays to emit light according to the stored energy of radiation, the emitted light is detected to obtain an image signal, and the obtained image signal is processed to improve the quality of a final visualized image of the object upon formation of the final image according to image processing data obtained by analyzing the image signal, and more particularly to an improved method of recording the image processing data for use in processing the image signal when recording the final image and an apparatus for carrying out the method.
2. Description of the Prior Art
There has been known a radiation image recording system in which a stimulable phosphor sheet is exposed to an imagewise radiation to store the energy of the radiation and record a latent image of a radiation image in the form of stored energy of radiation, and the stimulable phosphor sheet carrying the radiation latent image is exposed to stimulating rays to emit light according to the stored energy as disclosed in, for example, U.S. Pat. No. 3,859,527.
In the radiation image recording system, a finally read out image is recorded on a photosensitive material like a photographic film or displayed on a cathode ray tube (CRT). The image recorded on the photosensitive material or displayed on the CRT (which will be referred to as "final image" hereinbelow) should have high image quality and particularly should have high diagnostic efficiency and accuracy when the final image is a radiograph used for medical diagnosis. In order to obtain a desirable final image the read out image should preferably be electrically processed when recording the final image. For example, the stimulable phosphor carrying the radiation latent image is exposed to a laser beam or other stimulating rays to emit light according to the stored energy and the emitted light is detected to read out image information. The read out image information is converted to an electric signal and the obtained electric signal is analyzed to obtain image processing data such as information whether a low key or a high key should be used when recording the final image with the electric image signal, or whether the contrast of the radiation image is high or low. The analysis is carried out by calculating the maximum value, the minimum value and/or the average value, etc. from the electric image signal. The brightness, the contrast and the like of the final image may be controlled based on the result of the above analysis.
However, in order to carry out the above method when recording the final image, the whole information of the read out image must once be stored or memorized prior to said analysis. This inherently requires a memory unit having a large capacity. For example, the memory unit must be able to memorize a huge amount of information of 3.6.times.10.sup.7 bits assuming that an image of 30 cm.times.30 cm is detected through apertures of 50.mu..times.50.mu.. This prohibitively increases the manufacturing cost of the apparatus.
Further, this method will involve a substantial time lag, since said huge amount of information must be read out and the image analysis must be conducted using the read out information when recording the final image.
The inventors of the present invention have found that when a stimulable phosphor is exposed to a radiation, the stimulable phosphor stores a part of the energy of the radiation absorbed thereby and simultaneously discharges the rest of the energy as a spontaneous emission of light, and that the amount of the light momentarily emitted from the stimulable phosphor is proportional to the amount of the energy stored therein. Thus, information of the characteristics of the radiation image recorded on the stimulable phosphor in the form of stored energy of radiation can be obtained by detecting the spontaneous emitted light. The inventors have proposed a method of recording a final image in which image processing data are obtained through a calculation involving the information of the image characteristics obtained in the above manner and the image processing data are used for processing the electric image signal when recording the final image. (See U.S. patent application Ser. No. 80,310 now U.S. Pat. No. 4,284,889)
The information of the image characteristics can also be obtained simultaneously with recording the radiation latent image on the stimulable phosphor by exposing a detecting phosphor means like a phosphor plate or a number of phosphor members disposed behind the stimulable phosphor over the entire area thereof to the radiation transmitted through the object and and causing the phosphor to emit light. In this case, the material of the detecting phosphor means can be selected freely, and accordingly, an effective detection can be conducted by using a phosphor which can emit light of high luminance. The amount of the light emitted from the detecting phosphor means is also proportional to the amount of the energy stored in the stimulable phosphor. Therefore, the information of the image characteristics can be obtained by detecting the amount of the light emitted from the additional phosphor.
Each of the obtained image processing data must be accurately identified to correspond to each of the stimulable phosphor sheets for the data. This can be done by storing the obtained image processing data in a memory unit of a computer together with the serial number of the stimulable phosphor sheet corresponding thereto or by recording the image processing data on a recording medium together with the reference numeral of the corresponding stimulable phosphor sheet. However, these methods are disadvantageous in that the reference numeral or the serial number must be recorded on each of the stimulable phosphor sheets or the cassettes containing therein the phosphor sheet, which is troublesome. Further, the reference numerals or the serial numbers must be referred to when inputting the image processing data to the memory unit of the computer, during which miscorrespondence between the specific image processing data and the stimulable phosphor sheet is apt to occur.